Thermal power generation plays a major role in China's domestic power generation industry, the installed thermal power capacity being higher than 70%. The use of low-quality low-grade coals as power coals by most of thermal power plants causes the slagging on the water wall of a boiler furnace and the slagging and fouling on a convective heat-absorbing surface, which is one of the major problems affecting the normal running of the boiler in a power station. The slagging and fouling will reduce the heat transfer efficiency of the boiler, lower the output of the boiler and impair the operation security of a device, and a severe slagging may even lead to the flameout of a furnace, a pipe bursting, an unscheduled boiler shutdown and other serious accidents.
To avoid the various problems caused by fouling and slagging, a lot of research has been made on the mechanism of fouling and slagging by scholars at home and abroad and a plurality of slagging determination indexes have been proposed by the scholars which confront many limitations in the actual application and therefore only serve for a preliminary determination but cannot fundamentally eliminate the damages caused by contamination to a boiler. A method is proposed to regulate the combustion in the furnace of a boiler to control the temperature in the furnace to relieve the slagging problem of the boiler, this method, which cannot be operated conveniently in the actual application, is not popularized. For a high-alkalinity coal, the alkali metals volatilizing from the high-alkalinity coal are likely to condense on a boiler heating surface to form a bottom deposit which exists mainly in the form NaCl or Na2SO4. After volatilizing in a high-temperature environment, the foregoing components are likely to coagulate on a convective heat-absorbing surface to form a sintered or adhered ash deposit, the continuous absorption of the deposit to fly ash causes varying degrees of contamination to the convective heat-absorbing surface, moreover, the contaminants which cannot be removed using a soot blower reduce the heat transfer capability of the heat-absorbing surface, increase the temperature of the smoke discharged from the boiler and finally greatly reduce the output of the furnace of the boiler to shut down the boiler.
At present, there is a domestic lack of the engineering operation experience on the use of the combustion of a high-alkalinity coal, only several power plants in Xinjiang are studying the problem of the contamination caused by the combustion of a high-alkalinity coal but have not developed any effective high-alkalinity coal utilization method. The contamination problem can only be relieved through non-local coal blended combustion; non-local coal blended combustion is actually a method of reducing the relative content of the alkali metals contained in a raw coal by adding other low-alkalinity metal coals. The proportion of the high-alkalinity coal blended for combustion should be below 30%. When the proportion of the high-alkalinity coal blended for combustion is increased, the serious contamination caused by the ash deposit to the convective heat-absorbing surface generates a smoke passage, and the washout of smoke causes the leakage of a high temperature reheater and a high temperature superheater. As high-alkalinity coals are mainly used by electric power stations near coal-mines in Xinjiang, a high amount of non-local coals is needed for blended combustion, thus, this combustion mode is usually limited by transportation conditions and is therefore significantly increased in running cost. A platen superheater is arranged in the pulverized coal boiler of existing large power stations to reduce the outlet temperature of the furnace of the boiler and decrease molten slag, however, as relatively low in melting point, some alkali metal salts in smoke are still slagged when flowing through a convective heat-absorbing surface, the slagging phenomenon gets specifically worse in the combustion of Zhundong coal containing high-alkalinity metals. Advantaged in wide fuel applicability range, high combustion efficiency and few polluting emissions, circulating fluidized bed boiler has been rapidly developed in the past dozen years and widely commercially applied in the field of power station boilers. When a circulating fluidized bed burns a high-alkalinity coal as a power coal, the contamination to a convective heat-absorbing surface is also severe. The existence of slagging and fouling limits the large-scale efficient utilization of high-alkalinity coals and consequentially restricts the utilization efficiency of the energies of China.